Pulse laser resonator

ABSTRACT

A laser is disclosed which includes a gain medium, a switch element, and a pulse controller. In one embodiment laser light of differing polarizations pass along respective paths and a pulsed laser output is generated via an electro-optical element. In another embodiment light of differing polarizations passes in differing directions through a cyclical path. The invention can make use of a prism-shaped polarizer having a polarization selection face and two further faces. Yet further initial pulses can be controlled to reduce energy, for example by progressively increasing the period or amplitude of successive pulses. As a result an efficient and high power laser apparatus is realized.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 10/380,655, filed on Dec. 8, 2003 (the “'655 Application”),entitled “Pulse Laser Resonator,” which is a national phase filing under35 U.S.C. 371 of International Application Number PCT/GB01/04118, whichwas filed on Sep. 13, 2001, and published as International PublicationNumber WO 02/23683 A2 on Mar. 21, 2002 (the “'683 Application”), andwhich in turn claims priority from Great Britain Patent ApplicationNumber 0022481.6, filed on Sep. 13, 2000 (the “'481 Application”), fromGreat Britain Patent Application Number 0022482.4, filed on Sep. 13,2000 (the “'482 Application”), from Great Britain Patent ApplicationNumber 0022480.8, filed on Sep. 13, 2000 (the “'480 Application”), fromGreat Britain Patent Application Number 0022476.6, filed on Sep. 13,2000 (the “'476 Application”), and from Great Britain Patent ApplicationNumber 0022478.2, filed on Sep. 13, 2000 (the “'478 Application”). The'655 Application, the '683 Application, the '481 Application, the '482Application, the '480 Application, the '476 Application, and the '478Application are all assigned to the assignee of the present invention,and are all hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates generally to laser cavities, and moreparticularly to electro-optical laser cavities including at least oneelectro-optical (EO) switch element for cavity dumping a pulsed laseroutput of high average power, and a pulsed laser incorporating the same,as well as a polarizer for use in such cavities. This invention furtherrelates generally to the field of pulsed lasers, and more particularlyto a switched cavity for a pulsed laser incorporating an optical elementthat can be switched between a high loss state and a low loss state.

Electro-optical laser cavities include a polarization selective element,for example a polarizer, and an electro-optical switch element which isoperable to switch the cavity between low and high loss states,providing for hold off of the lasing action in the low loss (high Q)state and causing cavity dumping on switching to the high loss (low Q)state. The electro-optical element is operated, by the selectiveapplication of a voltage thereto, to alter the polarization state of thelaser light transmitted therethrough, with the degree of change in thepolarization state being related to the loss experienced by the cavity.Electro-optical elements have the particular benefit of being switchableat high rates to generate short laser output pulses, typically of theorder of nanoseconds down to femtoseconds.

Existing electro-optical laser cavities have been used to deliver laseroutput at low average powers, typically of the order of tens of watts asachieved by flash lamp pumping a gain medium. Flash lamp pumped lasercavities are not, however, suited to delivering laser output at higheraverage powers as significant heat is imparted to the gain medium, whichheating leads to thermal aberrations in the optical elements, inparticular the gain medium, and in turn causes the depolarization of thelaser light. This depolarization leads to loss at the polarizationselective element, rendering the laser cavity inefficient in convertingenergy stored in the gain medium into output pulses of laser light.Diode pumped laser cavities also suffer from the same problem whendriven to deliver laser output at higher average powers, typically aboutone hundred watts.

Modifications have been proposed to existing laser cavities in anattempt to compensate for the deleterious effects of polarizationbirefringence and allow use at higher average powers; see, for example,Op. Lett., 11, pages 360 to 362 (1986), Appl. Op., Vol. 32, No. 3 (1993)and IEEE J. Quan. Elec., Vol. QE-16, No. 4 (1980). However, thesemodifications have only been applied to flash lamp pumped laser cavitiesfor delivering output powers of tens of watts. These modificationsrequire the use of additional optical elements, in particularpolarization rotation optics, for correcting the polarizationbirefringence. It is, however, very difficult in practice to achieve therequired correction by utilizing polarization rotation, and also theintroduction of additional optical elements reduces the power of thepulsed laser output.

Another known arrangement is described in GB2037064. This document dealswith correction of the effects of thermal birefringence in a laser rodincluding a quarter-wave plate allowing rotation of the plane ofpolarization through 90 degrees after reflection. This arrangement,however, is an unnecessarily complex arrangement which has anundesirably low peak power and does not allow seeding of the arrangementfrom another source which in turn reduces the opportunities for a stableoperation of the laser.

A further arrangement is described in EP 0370620. According to thisarrangement a laser medium exhibiting thermal birefringence is providedin a branched cavity and the output is taken from one of the branchedarms in a Q-switching arrangement. This arrangement emits undesirablylow peak powers.

It is known to use acousto-optic elements within laser cavities toswitch high average power laser cavities to and from high and low lossstates in order to generate pulses of the laser radiation. Suchacousto-optic switching elements suffer from limitations in theshortness of the laser output pulses that can be produced, which in turnmay limit the maximum peak output power of the laser pulses. It is knownwhen using acousto-optic switching elements that measures should betaken to suppress generation of an excessively high power first pulsewhen switching on the laser as this may cause damage to the laser systemas a whole. This has been addressed by slowly ramping up the voltageapplied to the acousto-optic switching element during startup.

Measures that allow for a decrease in laser pulse duration withoutincreasing the risk of system failure through generation of anexcessively high energy laser pulse at startup are stronglyadvantageous. Known polarizers suffer from various problems. A polarizercan be used to separate in angle and spatially, polarizations of aninput laser beam. Polarizers are characterized in part by theirthreshold to damage. Polarizers suitable for high peak powerapplications are generally constructed from a plate of silica or glass,coated with a multilayer, thin film dielectric coating. The back surfaceof the plate is generally left uncoated. Such a design is optimum forthe separation of polarizations for a beam traveling in a pre-designateddirection through the plate. However in cavities where the beam travelsin both directions through the polarizing element, when the polarizationis rotated by a polarization rotator, the beam is ejected from thecavity. However with a standard plate polarizer, the beam alsoexperiences a significant reflection from the uncoated surface of theplate polarizer. This constitutes a mechanism of loss of energy from themain output beam.

One known solution to this problem is to use either a cube polarizer (orsimilar device), consisting of two triangular (or similar) sections, ora plate polarizer with a back surface coated with an anti-reflection(AR) coating for both p and s polarizations. Both of these options arehowever, complex. The AR coating for the plate polarizer is more complexthan standard coatings, and as such is both challenging to coatingmanufacturers and reduces the damage threshold of the device.

It is thus an aim of the present invention to provide improved lasercavities for cavity dumping a pulsed laser output of high average power.It is a further aim of the present invention to provide an improvedpolarizer for use in laser cavities. It is a particular aim of thepresent invention to provide a polarizer which can withstand a highpower laser and deliver a single output beam.

SUMMARY OF THE INVENTION

The disadvantages and limitations of the background art discussed aboveare overcome by the present invention. Accordingly, the presentinvention provides a laser cavity for generating a pulsed laser output,comprising: a gain medium, for example a diode pumped gain mediumincluding a light transmission axis; at least one first optical elementdisposed on one side of the gain medium to return laser light emittedfrom the one side of the gain medium back through the gain medium; apolarization selective element disposed on the light transmission axisto the other side of the gain medium; at least one second opticalelement arranged to define a first optical path from the polarizationselective element along which laser light of one polarization is in usedirected; at least one third optical element arranged to define a secondoptical path from the polarization selective element along which laserlight of other polarization is in use directed; and at least oneelectro-optical element disposed in one of the first and second opticalpaths and being operable to switch the polarity of the laser lighttransmitted therethrough such as to generate a pulsed laser output. Forexample first and second electro-optical elements may be disposed in therespective first and second optical paths.

In another aspect the present invention provides a laser cavity forgenerating a pulsed laser output, comprising: a gain medium, for examplea diode pumped gain medium including a light transmission axis; at leastone optical element disposed on one side of the gain medium to returnlaser light emitted from the one side of the gain medium back throughthe gain medium; a polarization selective element disposed on the lighttransmission axis to the other side of the gain medium; at least onefurther optical element arranged to define a cyclic optical path fromthe polarization selective element along which laser light is in usedirected in one of two directions depending on the polarization of thelaser light; and at least one electro-optical element disposed in theoptical path and being operable to switch the polarity of the laserlight transmitted therethrough such as to generate a pulsed laseroutput.

Preferred aspects of the invention are further set out in the claims.

Preferably the path between the gain medium and said first opticalelement is uninterrupted by polarization altering elements. As a resultthe design is significantly simplified.

Yet further preferably the electro-optical element is arranged to dumpthe cavity on operation. The cavity dumping arrangement allows anespecially high peak power output.

Preferably the first and second optical paths are detuned. Because thepaths are of different lengths the effects of interference between thetwo arms are substantially mitigated, especially where the cavity isoperated in the absence of seeding which improves the characteristics ofthe output beams in terms of spatial homogeneity and temporal anddirectional stability.

The invention in this preferred form does not correct for thermalbirefringence but instead provides a laser that has thermalbirefringence in the gain medium for a polarization switchedapplication. This enables a greater peak power to be emitted from thelaser, as well as enabling seeding of the laser with a pulsed orcontinuous wave laser source (regenerative amplification). This can leadto a more stable operation of the laser especially at high repetitionrates. The omission of a polarization altering element is particularlyadvantageous where the beam is many times (for example greater than tentimes) transform limited.

Preferably the pulsed laser output is output from the polarizationselective element. This arrangement allows cavity dumping and hencesurprisingly high peak powers. In addition a single output pulse can beobtained even if the cavity is seeded by another short pulsed lasersource.

The invention also extends to a polarizer including a first polarizationseparating optical surface, for example having a dielectric filmcoating, and second and third optical surfaces through which laser lightis in use transmitted.

The present invention also provides a laser cavity including theabove-described polarizer.

The polarizer is preferably prism-shaped, for example having atriangular cross-section. The triangular polarizer enables theapplication of a simple dielectric antireflection coating on twosurfaces, and a standard polarization separating coating on the other.The two surfaces that have a simple AR coatings are aligned near normalto the laser beam. The coating can be a single layer, and still reduceloss inducing reflections to a useable level. The polarizer can be usedin the various cavities described herein.

The present invention also provides a laser cavity for generating apulsed laser output, comprising: a gain medium including a lighttransmission axis; at least one optical element disposed on one side ofthe gain medium to return laser light emitted from the one side of thegain medium back through the gain medium; a polarization selectiveelement disposed on the light transmission axis to the other side of thegain medium; at least one further optical element arranged to define acyclic optical path from the polarization selective element along whichlaser light is in use directed in one of two directions depending on thepolarization of the laser light; and at least one electro-opticalelement formed of beta barium borate disposed in the optical path andbeing operable to switch the polarity of the laser light transmittedtherethrough such as to generate a pulsed laser output.

The present invention further provides a laser cavity for generating apulsed laser output, comprising: a gain medium including a lighttransmission axis; at least one first optical element disposed on oneside of the gain medium to return laser light emitted from the one sideof the gain medium back through the gain medium; a polarizationselective element, for example the above described polarizer, disposedon the light transmission axis to the other side of the gain medium; atleast one second optical element arranged to define a first optical pathfrom the polarization selective element along which laser light of onepolarization is in use directed; at least one third optical elementarranged to define a second optical path from the polarization selectiveelement along which laser light of other polarization is in usedirected; and at least one electro-optical element formed of beta bariumborate disposed in one of the first and second optical paths and beingoperable to switch the polarity of the laser light transmittedtherethrough such as to generate a pulsed laser output.

Viewed from one aspect the present invention provides a pulsed lasercomprising: a laser cavity; a laser gain medium within said lasercavity; an optical switch element operable to change a light loss levelof said laser cavity and laser gain medium between a first loss state inwhich laser output is inhibited and second loss state in which laseroutput is not inhibited such that a laser pulse is generated; and apulse controller operable to provide a control signal to said switchelement to control repeated switching between said first loss state andsaid second loss state to trigger generation of a stream of laserpulses; wherein upon initiation of generation of laser pulses, saidpulse controller generates control signals that progressively increase aperiod for which said laser cavity and laser gain medium are in saidsecond loss state or progressively change said cavity light loss levelof said second loss state towards a steady state second loss period suchthat laser pulse energy at startup is reduced compared to if said steadystate second loss period or level was used immediately upon startup. By“steady state” is meant a regular, non-changing period.

The invention recognizes that suppression of a potentially damagingfirst pulse may be achieved by progressively increasing the period forwhich the low (second) loss state is adopted up to a steady state lowloss period. In this way, the initial pulses produced are clipped suchthat the energy that they contain is reduced.

In preferred embodiments the minimum period of the second loss stateadopted at startup is too low to allow lasing to occur. In this way, asmooth and progressive startup of lasing may be aided.

While it will be appreciated that the technique of gradually increasingthe second loss state period may be applied to different sorts ofoptical switch elements, including both acousto-optic and electro-opticswitch elements, the invention is particularly well suited for use withelectro-optic switch elements as these are able to change state at highspeed and so achieve a controllable and repeatable clipping effect ofthe initial pulses.

While the optical switch elements could contain many differentcomponents, preferred embodiments are ones in which the optical switchelement comprises one or more of at least one mirror,polarization-modifying optics, focusing or diverging optics,beam-profiling optics, an electro-optic cell and an acousto-optic cell.These elements allow compensation for other characteristics of the lasersystem to be made while still providing the desired suppression ofinitial startup pulses having an excessive energy.

Viewed from yet another aspect the present invention provides a methodof operating a pulsed laser having a laser cavity, a laser gain mediumwithin said laser cavity, an optical switch element operable to change alight loss level of said laser cavity and laser gain medium between afirst loss state in which laser output is inhibited and a second lossstate in which laser output is not inhibited such that a laser pulse isgenerated, and a pulse controller operable to provide a control signalto said optical switch element to control repeated switching betweensaid first loss state and said second loss state to trigger generationof a stream of laser pulses; said method comprising the step of: uponinitiation of generation of laser pulses, generating control signalsthat progressively increase a period for which said laser cavity andlaser gain medium are in said second loss state towards a steady statesecond loss period such that laser pulse energy at startup is reducedcompared to the situation if said steady state second loss period wasused immediately upon startup.

Viewed from one aspect the present invention provides a pulsed lasercomprising: a laser cavity; a laser gain medium within said lasercavity; an electro-optic switch element operable to change a light losslevel of said laser cavity and laser gain medium between a first lossstate in which laser output is inhibited and second loss state in whichlaser output is not inhibited such that a laser pulse is generated; anda pulse controller operable to provide a control signal to saidelectro-optic switch element to control repeated switching between saidfirst loss state and said second loss state to trigger generation of astream of laser pulses; wherein upon initiation of generation of laserpulses, said pulse controller generates control signals thatprogressively change said cavity light loss level of said second lossstate towards a steady state second loss level such that laser pulseenergy at startup is reduced compared to the situation if said steadystate second loss level was used immediately upon startup.

The invention uses an electro-optic switch element as compared with anacousto-switch element. Such an electro-optic switch element operates ina fundamentally different physical manner to bring about its switchingand is able to produce much shorter laser pulses. However, in order toguard against excessively high energy pulses at startup, the inventionprovides a pulse controller that progressively decreases the low(second) loss state level from a startup low loss state level towards asteady state low loss state level in such a way that the laser pulseenergy at startup is reduced to inhibit excessively high energy initialpulses while not imposing a limitation upon the energy level of thesubsequent pulses that may be produced.

A particularly safe way in which the laser can be started up is one inwhich the electro-optic switch element is controlled at startup toprovide a light loss level that is too high even in the low loss stateto allow lasing and then progressively to decrease this low loss statelevel such that lasing will start in a controlled manner.

In addition to altering the loss level states between which theelectro-optic switch element is moved upon startup, preferredembodiments also act to control the period for which the electro-opticswitch element remains in the second loss state to gradually increaseupon startup in a manner that suppresses generation of high energyinitial pulses.

In preferred embodiments the suppression of excessively high energyinitial pulses is improved when the minimum initial duration of thesecond loss state at startup is sufficiently short to not allow a laserpulse to be generated. This low loss state duration can then begradually increased so that lasing is commenced in a controlled manner.

It will be appreciated that the electro-optic switch element could takemany forms. However, preferred levels of control and matching with othercharacteristics of a high-power system may be achieved when theelectro-optic switch element comprises one or more of at least onemirror, polarization-modifying optics, focusing optics, andbeam-profiling optics.

It will be appreciated that the pulse controller as well as serving tocontrol laser startup to avoid system damage may also be operable tocontrol the pulse frequency of the laser pulses that are generated.

Viewed from another aspect the present invention provides a method ofoperating a pulsed laser having a laser cavity, a laser gain mediumwithin said laser cavity, an electro-optic switch element operable tochange a light loss level of said laser cavity and laser gain mediumbetween a first loss state in which laser output is inhibited and asecond loss state in which laser output is not inhibited such that alaser pulse is generated, and a pulse controller operable to provide acontrol signal to said electro-optic switch element to control repeatedswitching between said first loss state and said second loss state totrigger generation of a stream of laser pulses; said method comprisingthe step of: upon initiation of generation of laser pulses, generatingcontrol signals that progressively change said cavity light loss levelof said second loss state towards a steady state second loss level suchthat laser pulse energy at startup is reduced compared to if said steadystate second loss level was used immediately upon startup.

DESCRIPTION OF THE DRAWINGS

These and other advantages of the present invention are best understoodwith reference to the drawings, in which:

FIG. 1 is a schematic illustration of a laser cavity in accordance witha first embodiment of the present invention;

FIG. 2 is a schematic illustration of a prism-shaped polarizer inaccordance with a preferred embodiment of the present invention;

FIG. 3 is a schematic illustration of a laser cavity in accordance witha second embodiment of the present invention;

FIG. 4 is a schematic illustration of a laser cavity in accordance witha third embodiment of the present invention;

FIG. 5 is a schematic illustration of a pulsed laser;

FIG. 6 is a schematic illustration of the generation of an excessivelyhigh energy laser pulse upon startup;

FIG. 7 is an illustration of a first embodiment of the present inventionby which control of the switching between a high loss state and a lowloss state of the cavity during startup can be used to suppressgeneration of an excessively high energy initial pulse;

FIG. 8 is an illustration of a second embodiment of the presentinvention by which control of the switching between a high loss stateand a low loss state of the cavity during startup can be used tosuppress generation of an excessively high energy initial pulse;

FIG. 9 is an illustration of a third embodiment of the present inventionby which control of the switching between a high loss state and a lowloss state of the cavity during startup can be used to suppressgeneration of an excessively high energy initial pulse; and

FIG. 10 is a schematic illustration of an electro-optical switchelement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a laser cavity in accordance with a first embodimentof the present invention for cavity dumping a pulsed laser output ofhigh average power.

The laser cavity includes a diode pumped gain medium 1 having a lighttransmission axis extending therethrough.

The laser cavity further includes a first optical element 3, in thisembodiment a high reflector, disposed on the light transmission axis toone side of the gain medium 1 to reflect laser light emitted from theone side of the gain medium 1 back through the gain medium 1.

The laser cavity further includes a polarization selective element 5, inthis embodiment a prism-shaped polarizer including coated opticalsurfaces 5 a, 5 b, 5 c, disposed on the light transmission axis to theother side of the gain medium 1. In this embodiment one, the first,optical surface 5 a faces the gain medium 1 and includes a thindielectric film coating 7. The other, second and third optical surfaces5 b, 5 c are each angled substantially normal to the laser lighttransmitted therethrough and include anti-reflective coatings 9, 9. In apreferred embodiment the bandwidth of the laser light is maintained suchas to be sufficiently narrow that dispersion by the polarizer 5 ofdiffering wavelengths in different directions does not impactsignificantly on the laser operation or the spatial beam quality. In analternative embodiment the polarization selective element 5 could be aplane parallel plate polarizer which includes a thin dielectric filmcoating on the optical surface facing the gain medium 1 and ananti-reflective coating on the other optical surface. Although a planeparallel plate polarizer could be used as the polarization selectiveelement 5, the prism-shaped polarizer is clearly advantageous over theplane parallel plate polarizer. Available information confirms that, ascompared to a standard plane parallel plate polarizer with ananti-reflective coating on the back surface, the damage threshold tolaser peak power density can be increased by about 20%, and, incomparison to a standard plane parallel plate polarizer without ananti-reflective coating on the back surface, the useful laser powertransmitted is increased by about 18%.

The laser cavity further includes a second optical element 11, in thisembodiment a high reflector, arranged to reflect laser light to or fromthe second optical surface 5 b of the polarizer 5.

The laser cavity further includes a third optical element 13, in thisembodiment a high reflector, arranged to reflect laser light to or fromboth the second optical element 11 and the first optical surface 5 a ofthe polarizer 5. In this embodiment the polarizer 5 and the first andsecond optical elements 11, 13 define a triangular optical path aboutwhich laser light is cyclically directed in the high Q state. It shouldbe understood, however, that other optical path shapes could beemployed. For example, a square optical path could be defined by theinclusion of a further optical element.

The laser cavity further includes an electro-optical switch element 15located on the optical path between the second and third opticalelements 11, 13 and being operable to switch, that is, rotate, thepolarization of the laser light transmitted therethrough through 90degrees. In this embodiment the electro-optical element 15 is formed ofbeta barium borate (BBO) which exhibits less thermal distortion and alsoenables higher switching rates as compared to the conventional materialsof KH₂PO₄ (KDP) and KD₂PO₄ (KD*P).

The laser cavity further includes a seed light source 17 operable in onemode of operation to seed the gain medium 1 with laser light, either asa pulse or as a continuous wave (CW). In this embodiment the lasercavity can be seeded through the polarizer 5. In other embodiments thelaser cavity could be seeded through or off any of the optical elements.In another mode of operation the gain medium 1 is driven from the noisetherein, referred to as spontaneous decay.

Operation of the laser cavity in cavity dumping a pulsed laser output ofhigh average power will now be described hereinbelow.

In one mode of use, the electro-optical element 15 is switched such thatthe laser cavity is in the high Q state, that is, the laser light istrapped in the cavity, and the flux of the laser light in the cavityincreases from noise. At or near the peak of the laser flux, theelectro-optical element 15 is switched, in a time period less than or ofthe order of the time period of the cavity round trip, such that thecavity is in the low Q state and a pulse of laser light is dumped fromthe cavity.

In another mode of use, the electro-optical element 15 is switched suchthat the laser cavity is in the high Q state and the cavity is seeded bythe seed light source 17. At or near the peak of the flux of the laserlight in the laser cavity, the electro-optical element 15 is switched,in a time period less than or of the order of the time period of thecavity round trip, such that the cavity is in the low Q state and apulse of laser light is dumped from the cavity. The advantages ofseeding the cavity are to improve the spatial beam quality, deliver anoutput pulse of shorter duration, and provide control of the outputpulse profile in time by enabling control of the seed input pulseprofile in time.

FIG. 3 schematically illustrates a laser cavity in accordance with asecond embodiment of the present invention for cavity dumping a pulsedlaser output of high average power.

The laser cavity includes a diode pumped gain medium 1 having a lighttransmission axis extending therethrough.

The laser cavity further includes a first optical element 3, in thisembodiment a high reflector, disposed on the light transmission axis toone side of the gain medium 1 to reflect laser light emitted from theone side of the gain medium 1 back through the gain medium 1.

The laser cavity further includes a polarization selective element 5, inthis embodiment a prism-shaped polarizer including coated opticalsurfaces 5 a, 5 b, 5 c, disposed on the light transmission axis to theother side of the gain medium 1. In this embodiment one, the first,optical surface 5 a faces the gain medium 1 and includes a thindielectric film coating 7. The other, second and third, optical surfaces5 b, 5 c are each angled substantially normal to the laser lighttransmitted therethrough and include anti-reflective coatings 9, 9. In apreferred embodiment the bandwidth of the laser light is maintained suchas to be sufficiently narrow that dispersion by the polarizer 5 ofdiffering wavelengths in different directions does not impactsignificantly on the laser operation or the spatial beam quality. In analternative embodiment the polarization selective element 5 could be aplane parallel plate polarizer which includes a thin dielectric filmcoating on the optical surface facing the gain medium 1 and ananti-reflective coating on the other optical surface. Although a planeparallel plate polarizer could be used as the polarization selectiveelement 5, the prism-shaped polarizer has clear advantages over theplane parallel plate polarizer. As mentioned hereinabove, availableinformation has confirmed that, as compared to a standard plane parallelplate polarizer with an anti-reflective coating on the back surface, thedamage threshold to laser peak power density can be increased by about20%, and, in comparison to a standard plane parallel plate polarizerwithout an antireflective coating on the back surface, the useful laserpower transmitted is increased by about 18%.

The laser cavity further includes a second optical element 11, in thisembodiment a high reflector, arranged to reflect laser light back to thesecond optical surface 5 b of the polarizer 5.

The laser cavity further includes a third optical element 13, in thisembodiment a high reflector, arranged to reflect laser light back to thefirst optical surface 5 a of the polarizer 5. In this embodiment thepolarizer 5 and respective ones of the second and third optical elements11, 13 define separate optical paths as ones of a fork. In thisembodiment the second and third optical elements 11, 13 are arrangedsuch that the path lengths of the optical paths differ i.e. are de-tunedfor example by the order of a few centimeters. The advantage ofarranging the optical paths to be of different length is thatinterference between the laser light of different polarity travelingthrough each optical path is avoided, leading to improved output spatialquality and temporal and direction stability. The path lengths can beclosely matched in laser cavities predisposed to higher output spatialquality, for example with less thermal loading in the gain medium 1,without immediate loss of spatial quality. In another embodiment thepath lengths could be the same.

The laser cavity further includes first and second electro-opticalswitch elements 15, 16 located in respective ones of the optical pathsbetween the second and third optical elements 11, 13 and the polarizer 5and each being operable to switch, that is, rotate, the polarization ofthe laser light transmitted therethrough through 90 degrees. In thisembodiment the electro-optical elements 15,16 are each formed of betabarium borate (BBO) which, as mentioned hereinabove, exhibits lessthermal distortion and also enables higher switching rates as comparedto the conventional materials of KH₂PO₄ (KDP) and KH₂PO₄ (KD*P).

The laser cavity further includes a seed light source 17 operable in onemode of operation to seed the gain medium 1 with laser light, either asa pulse or as a continuous wave (CW). In this embodiment the lasercavity can be seeded through the polarizer 5. In other embodiments thelaser cavity could be seeded through or off any of the optical elements.In another mode of operation the gain medium 1 is driven from the noisetherein, referred to as spontaneous decay.

Operation of the laser cavity in cavity dumping a pulsed laser output ofhigh average power will now be described hereinbelow.

In one mode of use, the electro-optical elements 15, 16 are switchedsuch that the laser cavity is in the high Q state, that is, the laserlight is trapped in the cavity, and the flux of the laser light in thecavity increases from noise. At or near the peak of the laser flux, theelectro-optical elements 15, 16 are switched, in a time period less thanor of the order of the time period of the cavity round trip, such thatthe cavity is in the low Q state and a pulse of laser light is dumpedfrom the cavity.

In another mode of use, the electro-optical elements 15, 16 are switchedsuch that the laser cavity is in the high Q state and the cavity isseeded by the seed light source 17. At or near the peak of the flux ofthe laser light in the cavity, the electro-optical elements 15, 16 areswitched, in a time period less than or of the order of the time periodof the cavity round trip, such that the cavity is in the low Q state anda pulse of laser light is emitted from the cavity. As mentionedhereinabove, the advantages of seeding the cavity are to improve thespatial beam quality, deliver an output pulse of shorter duration, andprovide control of the output pulse profile in time by enabling controlof the seed input pulse profile in time.

FIG. 4 illustrates a laser cavity in accordance with a third embodimentof the present invention for cavity dumping a pulsed laser output ofhigh average power.

The laser cavity of this embodiment is very similar to the laser cavityof the above-described second embodiment, and thus, in order to avoidunnecessary duplication of description, only the differences will bedescribed in detail.

This embodiment differs only in that the second and third opticalelements 11, 13 include stigmatism imparting optical elements, in thisembodiment astigmatic mirrors, and that the laser cavity furtherincludes fourth and fifth, stigmatism imparting optical elements 19, 21,in this embodiment astigmatic lenses, disposed in respective ones of theoptical paths between the polarizer 5 and the respective electro-opticalelement 15, 16. In the context of the present invention, it is meant bystigmatism imparting that a stigmatism is imparted to non-stigmaticlight and the stigmatism of stigmatic light is altered. In otherembodiments the stigmatism imparting optical elements could include onesof cylindrical or tilted lenses and mirrors. Operation of the lasercavity is the same as for the above-described second embodiment. Byseparately compensating for the differing astigmatism imparted by thegain medium 1 to the horizontal and vertical polarizations, a laseroutput is achieved which is free of astigmatism and does not require theuse of two matched gain media and an optical rotator, such as apolarization rotator formed preferably of quartz. The lattercompensation technique is generally considered to be incomplete whenapplied to real laser cavities in relying on the gain media havingidentical lensing characteristics and the optical ray passing theseparate gain media in exactly the same manner. As will be appreciated,these conditions are difficult to meet in real laser cavities.

FIG. 5 schematically illustrates a pulsed laser 2 having a laser cavity4. The laser cavity 4 is bounded by high reflectors 6 and 8. (The highreflector 8 could be replaced with a partial reflector in certaindesigns to stop the cavity flux becoming too high.) A pumped laser gainmedium 10 is disposed within the laser cavity 4. An electro-optic switchelement 12 is also disposed within the laser cavity 4. The electro-opticswitch element 12 also includes a polarization element which can be usedto direct radiation out of the laser cavity 4 when it is desired to“dump” the cavity. A pulse controller 14 coupled to the electro-opticswitching element 12 serves to switch the electro-optic switchingelement 12 between a high loss state and a low loss state.

In the case of the use of an acousto-optic switch element (which createsa diffraction grating from internal acoustic waves that diffracts theradiation so causing a loss) in place of the electro-optic switchelement 12, a partial reflector would replace the high reflector 8 toprovide an exit for radiation from the laser cavity 4.

In steady state operation, the high loss state of the electro-opticswitch element 12 is such that lasing within the laser cavity 4 isinhibited while the low loss state of the electro-optic switch element12 is such that lasing within the laser cavity 4 will occur. The rapidresponse of the electro-optic switch element 12 in moving between thesetwo different loss states enables very short laser pulses to begenerated.

In the case of the use of the electro-optic switching element 12, highlyreflecting mirror 6 and 8 are used at both ends of the cavity and thereis the added advantage of being able to seed and dump the cavity toproduce shorter pulses. If built up from laser noise, there is aconstant laser flux everywhere in the laser cavity 4 until theelectro-optic switch element is switched, when the cavity flux can bedumped (output) in a round trip time, hence creating an output pulse. Ifseeded, a pulse goes around in the cavity until one chooses to dump itout, which may enable an even shorter laser pulse to be produced.

FIG. 6 schematically illustrates the possibility of laser damage thatcan occur with an uncontrolled startup. In the example illustrated, thefirst laser pulse 16 that is generated may have a peak intensity/powerthat is sufficient to produce damage to the system as a whole. Thisexcessively high power level of the first pulse may be the result of agreater population inversion within the laser gain medium 10 at startupthan in the steady start or other characteristics of the pulsed lasersystem 2 that vary at startup compared to the steady state.

As shown in the example, subsequent laser pulses generated are below thelevel at which damage may occur and eventually settle to aquasi-constant level as the pulsed laser system 2 equalizes to itssteady state condition.

FIG. 7 illustrates a first technique by which the pulse controller 14can modify the control pulses (control voltages, which may be negative)supplied to the electro-optic switch element 12 in a manner that reducesthe likelihood of generation of an excessively high energy initialpulse. As shown in the lower graph, the loss level associated with theelectro-optic switch element 12 is switched between a high loss level HLand a low loss level beneath this. The high loss level HL is above alasing threshold such that lasing is inhibited by the degree of losswithin the laser cavity 4 when the electro-optic switch 12 has this highloss level HL. At startup, the first low loss level state adopted has alevel L₀ beneath the high loss level HL. This first low loss level stateis insufficiently low to allow lasing to occur. Subsequent pulsed lowloss level states adopted decrease the low loss state levelprogressively through the lasing threshold such that lasing graduallybecomes sustainable until a steady state low loss level state L₃ isreached. In the example illustrated, the progressive decrease in the lowloss level is shown as occurring relatively rapidly whereas in practicethis may take place over many more laser pulse cycles.

FIG. 8 illustrates an alternative way in which the pulse controller 14may generate control pulses that suppress the generation of anexcessively high energy initial pulse. In this embodiment the period forwhich the control pulse replaces the electro-optic switch element 12 inthe low loss state is progressively increased from a period t₀ to asteady state period t₃. The initial low loss state period t₀ isinsufficient to allow lasing to occur. As this period graduallyincreases, laser flux starts to build up, but is clipped in comparisonto what would be their normal duration by return of the laser cavity 4to a high loss state using the electro-optic switch 12 prior to theenergy stored in the laser gain medium 10 being depleted by thegeneration of the laser pulse. Again, the increase in the low loss stateperiod illustrated in FIG. 4 takes place over many fewer laser pulsecycles than may in practice be utilized.

FIG. 9 illustrates a third embodiment in which a combination of thetechniques of FIGS. 7 and 8 is used. In this embodiment both the levelof the low loss state is progressively decreased and the duration of thelow loss state progressively increased in a manner whereby lasing isinitially not sustainable and then progressively becomes sustainable ina manner in which the maximum pulse energy is not sufficient to renderdamage to the laser system 2.

FIG. 10 schematically illustrates an example electro-optic switchingelement 12 in more detail. This electro-optic switch incorporates anelectro-optic cell 18, containing a material that exhibits the Pockelseffect (such as KDP, LiNO₃ or BBO), and a polarization selective element20. The electro-optic switch switches the polarization of light passingthrough it in dependence upon a control voltage applied by the pulsecontroller 14.

An electro-optic cell can switch slowly (order of many round trips), inwhich case it can be used in a similar way to an acousto-optic switch.There is still an advantage of an electro-optic switch though, in thatit can hold off greater cavity gain from lasing.

If switched fast, electro-optic cells enable short pulse production (afew nanoseconds is possible), even for high average power laseremissions. They also enable seeding. Seeding may give the advantage ofimproved spatial beam quality as well as a shorter output pulse.

It will be appreciated that in the embodiments of FIGS. 4 and 5, theelectro-optic switch 12 could be replaced by an acousto-optic switchusing the same control techniques to stop damaging pulses on start up.

In one modification of the laser cavity of the above-described firstembodiment, the single electro-optical element 15 could be replaced bytwo electro-optical elements separated by an optical rotator, typicallya polarization rotator preferably formed of quartz. This arrangementcompensates for the polarization birefringence within electro-opticalelements and also partially within the gain medium 1.

In one modification of the laser cavity of the above-described secondembodiment, one of the electro-optical elements 15, 16 could be omittedfrom one optical path. In operation, the laser light of the polaritywhich travels through the one optical path including no electro-opticalelement is eventually depolarized sufficiently by the gain medium 1 asto travel through the other optical path and be controlled by theelectro-optical element in that other path either to remain in the lasercavity or be switched out. This modified laser cavity offers theparticular advantage that the laser output is of a single polarizationstate. A disadvantage is that the output pulse may be longer induration.

Although the foregoing description of the pulse laser resonator of thepresent invention has been shown and described with reference toparticular embodiments and applications thereof, it has been presentedfor purposes of illustration and description and is not intended to beexhaustive or to limit the invention to the particular embodiments andapplications disclosed. It will be apparent to those having ordinaryskill in the art that a number of changes, modifications, variations, oralterations to the invention as described herein may be made, none ofwhich depart from the spirit or scope of the present invention. Theparticular embodiments and applications were chosen and described toprovide the best illustration of the principles of the invention and itspractical application to thereby enable one of ordinary skill in the artto utilize the invention in various embodiments and with variousmodifications as are suited to the particular use contemplated. All suchchanges, modifications, variations, and alterations should therefore beseen as being within the scope of the present invention as determined bythe appended claims when interpreted in accordance with the breadth towhich they are fairly, legally, and equitably entitled.

1. A laser cavity for generating a pulsed laser output, comprising: again medium including a light transmission axis; at least one firstoptical element disposed on one side of the gain medium to return laserlight emitted from the one side of the gain medium back through the gainmedium; a polarization selective element disposed on the lighttransmission axis to the other side of the gain medium; at least onesecond optical element arranged to define a first optical path from thepolarization selective element along which laser light of onepolarization is in use directed; at least one third optical elementarranged to define a second optical path from the polarization selectiveelement along which laser light of other polarization is in usedirected; and at least one electro-optical element disposed in one ofthe first and second optical paths and being operable to switch thepolarity of the laser light transmitted therethrough such as to generatea pulsed laser output; wherein the at least one first optical elementcomprises a reflective element; wherein the polarization selectiveelement comprises a polarizer; wherein the polarizer includes a firstoptical surface which includes a dielectric film coating and is inoptical communication with the other side of the gain medium and the atleast one third optical element, a second optical surface which is inoptical communication with the at least one second optical element, anda third optical surface which is the surface through which laser outputpulses are in use dumped; and wherein the second and third opticalsurfaces of the polarizer are arranged such that the laser lighttransmitted therethrough is not more than ten degrees from a directionnormal to the respective optical surface.
 2. A laser cavity as definedin claim 1, wherein the at least one electro-optical element is formedof beta barium borate.
 3. A laser cavity as defined in claim 1, whereinthe polarizer comprises a plane parallel plate polarizer.
 4. A lasercavity as defined in claim 3, wherein the polarizer includes ananti-reflective coating on the back surface thereof.
 5. A laser cavityas defined in claim 1, wherein the polarizer comprises a prism-shapedpolarizer.
 6. A laser cavity as defined in claim 1, wherein the secondand third optical surfaces of the polarizer are arranged such that thelaser light transmitted therethrough is substantially normal to therespective optical surface.
 7. A laser cavity as defined in claim 1,wherein the second and third optical surfaces of the polarizer includean anti-reflective coating.
 8. A laser cavity as defined in claim 1,further comprising at least one further electro-optical element disposedin the other of the first and second optical paths and being operable inconjunction with the at least one electro-optical element to switch thepolarity of the laser light transmitted therethrough.
 9. A laser cavityas defined in claim 8, wherein the at least one further electro-opticalelement is formed of beta barium borate.
 10. A laser cavity as definedin claim 1, wherein at least one of the first and second optical pathsincludes at least one stigmatism imparting optical element.
 11. A lasercavity as defined in claim 10, wherein at least one of the at least onesecond and at least one third optical elements is a stigmatism impartingoptical element.
 12. A laser cavity as defined in claim 10, wherein boththe first and second optical paths include at least one stigmatismimparting optical element.
 13. A laser cavity as defined in claim 12,wherein the at least one second and at least one third optical elementsare stigmatism imparting optical elements.
 14. A laser cavity as definedin claim 1, wherein the at least one second optical element comprises areflective element.
 15. A laser cavity as defined in claim 1, whereinthe at least one third optical element comprises a reflective element.16. A laser cavity as defined in claim 1, further comprising a seedlight source for seeding the cavity with laser light.
 17. A laser cavityas defined in claim 1, wherein the gain medium is a diode pumped gainmedium.
 18. A laser cavity as defined in claim 1, wherein the pathbetween the gain medium and said first optical element is uninterruptedby polarization altering elements.
 19. A laser cavity as defined inclaim 1, wherein the electro-optical element is arranged to dump thecavity on operation.
 20. A laser cavity as defined in claim 1, whereinthe first and second optical paths are de-tuned.
 21. A laser cavity asdefined in claim 1, wherein the pulsed laser output is output from thepolarization selective element.
 22. A laser cavity for generating apulsed laser output, comprising: a gain medium including a lighttransmission axis; a first reflective element disposed on one side ofthe gain medium to return laser light emitted from the one side of thegain medium back through the gain medium; a polarizer disposed in thelight transmission axis on the other side of the gain medium, thepolarizer having first, second, and third optical surfaces, the firstoptical surface including a dielectric film coating and being in opticalcommunication with the other side of the gain medium, the third opticalsurface being the surface through which laser output pulses are dumped;a second reflective element arranged to define a first optical path fromthe polarizer along which laser light of one polarization is directed,the second optical surface being in optical communication with thesecond reflective element; a third reflective element arranged to definea second optical path from the polarizer along which laser light ofanother polarization is directed, the first optical surface also beingin optical communication with the third reflective element; and at leastone electro-optical element disposed in one of the first and secondoptical paths and being operable to switch the polarity of the laserlight transmitted therethrough to generate a pulsed laser output.
 23. Alaser cavity as defined in claim 22, wherein the second and thirdoptical surfaces of the polarizer being arranged such that laser lighttransmitted therethrough is not more than ten degrees from a directionnormal to the respective optical surface.
 24. A laser cavity as definedin claim 22, further comprising at least one further electro-opticalelement disposed in the other of the first and second optical paths andbeing operable in conjunction with the at least one electro-opticalelement to switch the polarity of the laser light transmittedtherethrough.